Field of the Invention
The present invention relates to a switching circuit including a pulse modulator.
Description of the Related Art
In order to amplify an audio signal and to drive a speaker, a Class D amplifier (switching amplifier) is employed. FIG. 1 is a circuit diagram showing an audio system 1r including such a Class D amplifier.
The audio system 1r includes an audio amplifier integrated circuit (which will be referred to as the “audio amplifier IC” hereafter) 300r, an output filter 106, and a speaker 108. The audio amplifier IC 300r mainly includes a switching output stage 301 and a pulse signal generator 303.
The pulse signal generator 303 generates a pulse modulated signal S5 according to an input audio signal S1. The switching output stage 301 includes a so-called Class D amplifier 310 and a pre-driver 308, and generates a switching pulse S2 according to the pulse modulated signal S5. The switching pulse S2 is input to the speaker 108 via the output filter 106. The switching signal S2 is converted by means of the output filter 106 into an audio signal S3 in the form of an analog signal.
A pulse modulator 306 generates the pulse modulated signal S5 having a duty ratio that is adjusted according to a signal S4 corresponding to the input audio signal S1. For example, the pulse modulator 306 is configured as an analog PWM (pulse width modulation) circuit including an oscillator 326 and a PWM comparator 328. The oscillator 326 generates a carrier signal S6 having a triangle waveform or otherwise a sawtooth waveform. The PWM comparator 328 compares the carrier signal S6 with the signal S4 corresponding to the input signal S1, so as to generate the pulse modulated signal S5. The pre-driver 308 drives the Class D amplifier 310 according to the pulse modulated signal S5 thus generated.
The switching pulse S2 has a voltage amplitude corresponding to a power supply voltage VDD of the class D amplifier 310. Accordingly, if there is a fluctuation in the power supply voltage VDD, this leads to fluctuation in the amplitude of the analog audio signal S3. This leads to noise or otherwise fluctuation in the volume. In order to reduce the effect of the power supply voltage VDD, an error amplifier 320 is provided. The error amplifier 320 includes a phase compensating filter 322, an operational amplifier 324, and a resistor R1. The phase compensating filter 322 is configured as a low-pass filter, and generates a feedback signal S7 according to the switching pulse S2. The operational amplifier 324 generates the error signal S4 corresponding to a difference between the input signal S1 and the feedback signal S7.
FIGS. 2A and 2B are diagrams each showing frequency characteristics of the audio system 1r shown in FIG. 1. FIG. 2A shows the loop gain of the audio system 1r. FIG. 2B shows the THD (total harmonic distortion). Assuming that the loop gain of the audio amplifier IC 300r is designed such that it is optimized for a carrier signal S6 having a first frequency fOSC1, in this case, after the loop gain is determined, the THD characteristic is determined according to the loop gain thus designed.
In some cases, depending on the usage of the audio system 1r, there is a need to change the frequency of the carrier signal S6. Even if the audio amplifier IC 300r optimized for the first frequency fOSC1 as shown in FIGS. 2A and 2B is operated with a second frequency fOSC2 that is higher than the first frequency fOSC1, the loop gain remains at the same level in the audio frequency band fA. It is needless to say that the THD remains at the same level.